The present invention relates to a stabilized dispersion magnetorheological fluid having a good dispersion stability and recovery of magnetorheological characteristic, both of which are sustained over a protracted period of time.
Liquid compositions which undergo changes in rheological characteristics in response to a magnetic field, which are generally called magnetorheological fluids, magnetic fluids, or magnetorheological materials, are already known. For example, an early description of magnetic field-responsive magnetic material-containing liquid compositions can be found in AIEE Transactions, xe2x80x9cCharacteristics of Magnetic Fluidsxe2x80x9d (published February, 1955), the paper 55-170 authored by J. D. Coolidge Jr. and R. W. Harberg (pp. 149-152). A magnetorheological fluid comprising iron oleate as a dispersing agent is disclosed in U.S. Pat. No. 2,661,596, and several relevant technologies are also disclosed in U.S. Pat. No. 3,006,656, U.S. Pat. No. 4,604,229, Japanese Kokai Publication Sho-51-13995, Japanese Kokai Publication Sho-51-44579 and so forth. These are invariably concerned with disperse systems such that magnetic particles (mean particle diameters: a few nm to tens of xcexcm) contained are oriented by externally applied magnetic fields to form chain-like clusters and accordingly gain in viscosity or form gels and, hence, show marked changes in rheological characteristics and yield stress.
The hitherto-proposed field of application of such magnetorheological fluids or magnetic fluids includes bearings, seal materials, centering devices, loudspeakers, clutches, brakes, dampers, shock absorbers, engine mounts, elevator functional members, and aseismatic structures for buildings.
However, none of these fluids have been commercially implemented for applications requiring comparatively large changes in rheological characteristics and yield stress, such as clutches, brakes, dampers, shock absorbers, and aseismatic structures for buildings.
In order that a magnetorheological fluid may express such characteristics as mentioned above, it is necessary that magnetic particles have been uniformly dispersed in a dispersive medium. However, since the true density of magnetic particles is considerably higher than that of the dispersive medium, it has been difficult to impart good dispersion stability to magnetorheological fluids. It is generally effective to increase the viscosity of the dispersive medium in order to improve the dispersion stability of a magnetorheological fluid but excessively increasing the viscosity of the dispersive medium leads to an increased viscosity of the magnetorheological fluid itself, thus giving rise to the problem that the magnetorheological fluid obtained is difficult to handle.
In the above state of the art, the present invention has for its object to provide a stabilized dispersion magnetorheological fluid which harbors magnetic particles of high specific gravity dispersed uniformly without aggregation or precipitation/agglomeration over a protracted period of time and can permanently keep good dispersion stability and recovery of magnetorheological characteristics.
The present invention is directed to a stabilized dispersion magnetorheological fluid comprising a magnetic particle, a dispersive medium and a dispersing agent
wherein said dispersing agent is a polyether containing a polymer unit (hereinafter referred to sometimes as polymer unit (1)) represented by the following general formula (1): 
in the formula, R1 represents a hydrogen atom or a hydrocarbon group of 1 to 22 carbon atoms; R2 represents a hydrogen atom, a hydrocarbon group of 1 to 22 carbon atoms which may optionally have a substituent, or xe2x80x94(AO)m-R3; R3 represents a hydrocarbon group of 1 to 22 carbon atoms which may optionally have a substituent; A represents an alkylene group of 1 to 3 carbon atoms; m represents an integer of 1 to 50; and A groups which number m may be the same or different. The dispersive medium for use in the present invention is preferably a hydrocarbon oil.